Spinels have been produced by adjusting the pH of an acidic aqueous solution of a mixture of a water-soluble magnesium salt and a water-soluble aluminum salt with an alkali to form a precipitate of hydroxides and then heating the precipitate to thereby synthesize a spinel, i.e., the coprecipitation method.
As an example of the above-described coprecipitation method, JP-A-59-232915 discloses "a method for synthesizing a spinel by adjusting the pH of an aqueous solution of a mixture of a water-soluble magnesium salt and a water-soluble aluminum salt with an alkali in the presence of alcohol to form a precipitate and then drying and calcining this precipitate to thereby synthesize a spinel" (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
In this method, the coprecipitation is performed by adjusting the pH in the presence of an alcohol so as to give a spinel powder in the form of fine grains having a low cohesiveness and a low crystallinity.
The "water-soluble magnesium salt" and the "water-soluble aluminum salt" employed here as the starting materials include chlorides, sulfates and nitrates of magnesium or aluminum, and acetate, carbonate and oxalate of magnesium, etc. It is disclosed that chlorides are preferably employed therefor, since they are available in a highly pure state and compounds contaminating the hydroxide precipitate can be easily washed away in this case.
As techniques for producing catalysts, on the other hand, it is known to form a catalytic layer on the surface of a catalytic base material, for example, a honeycomb construction by preparing a slurry containing a noble metal and a powdery support such as a spinel powder and an alumina powder, and applying this slurry onto the catalytic base material (see, JP-A-3-131343, JP-A-7-328454, JP-A-10-99698).
Recently, exhaust controls become more and more strict. With an increase in the regulation level, it has been required to achieve an enhanced catalytic activity. Although the spinel powder obtained by the above-described coprecipitation method has a low cohesiveness, a low crystallinity and a high purity, a catalyst with the use of this spinel powder as a support cannot achieve a satisfactory catalytic activity, thus failing to satisfy the above requirement.
When a slurry containing a catalytic support is applied onto a catalytic base material in the catalyst-producing technique as described above, it is required that the slurry can be easily applied on and form a catalytic layer scarcely peeling off and being free from cracking.